In general, in a shield driving method, any collapsed condition of the natural ground on the outer surface of the shield machine is detected from the viewpoint of, for instance, controlling the charge amount of a back filling. A resistivity method has been known as a method for such detection. For instance, Japanese Utility Model Laid-Open No. 1-131186 discloses a method in which a row of Wenner electrodes are provided on an outer surface of the shield machine, resistivities differing between various strata are detected, and the thickness of a sludge layer forming the first stratum is calculated on the basis of changes in the resistivities, thereby detecting a collapsed condition of the natural ground.
With this method, however, since the values of the measured resistivities are directly used, the characteristics of the electrodes influence the calculated thickness of the sludge layer even when the characteristics of the electrodes have deteriorated with the passage of time. Thus, it has been impossible to detect the thickness of a stratum with a high level of precision. In addition, with the method (described in the above-identified publication) in which a row of Wenner electrodes are employed, resistivity measurement is possible only at a point at a certain depth basically determined by the distance between the electrodes. When multi-point measurement is to be performed, a large number of electrodes must be used. However, when a large number of electrodes are used, it is necessary that the position of the electrode on the side of the power supply source be successively changed. Thus, operational efficiency is greatly deteriorated In addition, the fact that the current supplied to the stratum cannot be easily stabilized further deteriorates efficiency. For these reasons, the disclosed method has too many problems to be applied to a shield driving method which requires such a measurement operation to be performed within a short period.
Furthermore, it is very difficult to determine the configuration of a collapse with the above method.